Electrodepositing lead on copper from a nitrate bath



Patented Oct. 18, 1949 ELECTRODEPOSITING LEAD ON COPPER FROM A NITRATE BATH Thomas S. Chambers, Newark, and Ober 0. Slotterbeck, Rahway, N. J., assignors to Standard Oil Development Company, a corporation of Delaware N Drawing. Application January 3, 1944, Serial No. 516,844

1 Claim.

The present invention pertains to a method of preparing cathodes for use in the electrolytic reduction of ketones and generally in all types of carbon to carbon reductive condensation reactions.

Specifically, our invention is concerned with the preparation of cathodes which are particularly suitable for the reduction of acetone electrolytically to sym-tetramethyl-ethylene glycol- It is known that the electrolytic reduction of ketones can take place at the cathode surface according to any one of the following reactions:

Other side reactions result in the formation of metal alkyls, especially when the reduction is eflected in acid medium with lead cathodes.

An electrolytic method for effecting the reductive condensation reaction whereby acetone is converted to pinacol (sym-tetramethylethylene glycol) using a lead cathode and an acid catholyte is described in German Patent 113,719 (1899). The claims of the German patent were verified by Elbs in Z. Electrochemie, '7, 644 (1901).

Several German patents pertaining to the electrolytic reduction of acetone to pinacol were issued in the period 1912-1920. Among these patents were DRP. 306,304 (1917), 306,523 (1918) and 324,919 (1920). The principal developments made in this period appear to have the use of mixed metal and alloy cathodes, such as 4 to copper-96 to 90% lead mixtures and 10% tin-90% lead alloy, and the use of horizontally suspended cathodes to prevent the metal alkyls formed from accumulating on and decreasing the activity of the electrode surface.

All of the prior processes for electrolytically converting acetone to pinacol have been unsatisfactory either because pinacol current efiiciency or the overall current efficiency or both were too low. The current efiiciency (C. E.) is the ratio of the theoretical quantity of electricity required for a given reduction to that actually used. Thus, the per cent pinacol grams of pinacol hydrate 53.6 mol. wt. of pinacol hydrateXtotal ampere hours It is the object of the present invention to provide the art with a novel method of preparing electrodes, for effecting the electrolytic reduction of organic compounds.

It is also the object of this invention to provide the art with a method of preparing lead coated cathodes of high activity in carbon to carbon reductive condensation reactions.

These and other objects will appear more clearly from the detailed description and claims which follow.

We have now found that by electrodepositing a thin film of lead on a base of low hydrogen overvoltage metal from a lead salt electrolyte containing a low concentration of other metal ions it is possible to obtain cathodes which minimize the formation of toxic lead alkyls that tend to diminish the effectiveness of the cathode and effect the reduction of ketones to 04.5 glyools with good current efiiciency and high ratio .of glycols to other reduction products. The metal ion content of the electrolyte may be provided by adding up to about 25 gm. per liter of a salt thereof to the electrolyte or may be advantageously formed in situ by making a sheet of the metal the anode in the electrolyte for a brief period and then reversing polarity of the cell making the metal plate the cathode and electrodepositing a thin layer of lead thereon. Specifically, we have found that cathodes consisting of a base of copper having a thin film of lead electrodeposited thereon from an electrolyte containing a low concentration of copper ions are particularly effective.

The metals of low hydrogen overvoltage that may be used as the base for our cathode include copper, nickel, silver, gold and platinum or alloys such as Monel metal and brass.

The film .of lead is preferably electrodeposited on the low overvoltage metal base in the following manner: Plates of the metal which are to serve as the base of the cathode are treated for a few minutes with one part of nitric acid to three parts of distilled water. The plates are washed with water to remove nitric acid and are then ready for lead plating. The plating bath contains about 50 to 100 grams of lead nitrate per liter of distilled water. Lead sheets are used as the other electrodes and upon assembly of the cell, the plates which are to receive the lead plating are made the anodes and the lead sheet is made the cathode at a current density of about 1.55 amperes/sq.dm. for minute. The 'current flow is thenreversed sothat the plates are cathodes and the lead sheets the anodes. Plating is continued for about 2 minutes at about the same or at a higher current density. The lead .coated plates are then removed from the plating bath and immediately washed with hot water and dried, whereupon they may be used directly without any further conditioning treatment. Alternatively, relatively small amounts of metal salts other than lead salts are added to the electrolyte whereupon the lead is electiiodeposited ion the base metal without any reversal of polarity f the base.

It is to be understood that this method 'or preparing our lead coated cathodes is merely illustrative and that other-methods can be utilized. For example, the lead can be deposited on the base plates from fluosilicate, fluoborate and perchlorate baths. The important factor in the preparation of the cathode is that the lead deposit must be kept very thin, preferably within the range of from about -0;02 to about 0.5 .gram/sq. dm.

The following examples serve to illustrate our invention but it is to be understood that our invention is not limited thereto.

EXAMPLE 1 Copper sheets about 21 x 20 cms. an'dabout 0.2 cm. in thickness were treated for a few minutes with a mixture of one part of nitric acid to'three parts of distilled water. The copper sheets were washed and then placed in a solution of lead nitrate containing 100 grams of lead nitrate/liter and made the anode while a leadorcarbon electrode was made the-cathode. Current was passed through the resulant cell for minute at a current density of about 1155 amperes/sq. dm. for minute. The polarity of the electrodes was then reversed, making the copper sheet the cathode and current was passed at a current density of about4.8-amperes per sq. dm. for 2120 3 minutes.

The resultant lead coated copper electrode was then used as the cathode in a diaphragm cell wherein the catholyte consisted of a 4/1 acetone -20% sulfuric acid solution. The anolyte was sulfuric acid. The electrolysis was conducted at 0-10 C. at a current density of 1.55 to 2 amperes/sq. dm. The run was conducted for 21 hours.

Upon conclusion of the electrolysis, isopropyl alcohol and unreacted acetone were stripped from the catholyte under reduced pressure at low temperatures (below 25 0.). The pinacol (symtetramethylethylene glycol) was seperated from the residue as the hexahydrate by cool n and filtering the crystals.

656 grams of pinacol hydrate corresponding to 343 grams of sym-tetramethylethylene glycol and 14 grams of isopropyl alcohol were obtained.

This corresponds to an overall current efliciency of 37% based on pinacol and 315% based on isopropyl alcohol. The mol ratio of pinacol hydrate to isopropyl alcohol was 1'0/1.

tions.

When using lead or 4% copper-96% lead, 01 10% copper-% lead mixtures in sheet form as the cathode in lieu of the lead deposited on copper cathodes with the same anolyte and catholyte and the same conditions of temperature and current density, current efficiencies based on pinacol of 12.7% and 13.8%, respectively, and a mol ratio of pinacol hydrate/isopropyl alcohol of 0.4/1 and 0.9/1, respectively were obtained.

A number of cathodes were prepared by electrodepositing lead onto copper plates in accordance with the present invention.

The firstcathode in the table set out below was prepared by arranging a copper sheet and a carbon electrode in a bath containing 100 grams per-.liter of lead nitrate and no copper. The coppersheet was made anode and the carbon cathode for less than a -minute whereupon the polarity was reversedand lead was deposited on the copper.sheet cathode at a current density of about 1 amp/sq. dm. for a period of 10 minutes.

The other electrodes listed in the table were prepared from electrolytes containing 100 grams per liter of lead nitrate and different amounts of other metal salts. These electrodes were prepared without reversing the polarity of the copper sheet, the current .density used in each case being the same as above, i. e. 1 amp/sq. dm.

The several cathodes were then "used for the reduction of acetone to pinacol and pinacolone in a diaphragm cell under the following condi- The anode used was made of chemical lead and had an area of "79 sq. dm. while the several cathodes used had an area of 258 sq. dm. each.

The catholyte volume was 830 c. c. and consisted of acetone and 30 wt. per cent sulfuric acid in a 3:1 ratio. The anolyte volume was 63 c. c. and consisted of .30 wt. per cent sulfuric acid. The current density used in the electrolysis was 1.0 amp/sq. dm. except in the first case wherein a current density of 1.6 amp/sq. dm. was used, other experiments having shown that no substantial alteration in current efiiciency is brought about by increasing current density from 1.0 to 1.6 amps/sq. 'dm. The electrolysis was effected at a temperature between 19 and 23 C. The quantity of reduction products formed was determined and the current efficiency based on pinacol and ,pinacolone was determined. The re- .sults obtained are summarized in the following .table:

Table Current Elec- .clency trode Salt Added to Electrolyte 21 3N0 .pmacol-and 1 pinacolone) Per cent None (Reversal of Polarity Technique Used) 46 l g./l. cupr'ic nitrate 40. 5 5.g./l. cupricnitrate 41. 5 l0 g.ll. cupric nitrate 46. 5 l5 g./l. cupric nitrate '48. 0 20 g./l. cupric nitrate 44. 0 l5 g. ll. cupric nitrate; 5 g./l.-mercuric nitrate 44. 5

The ketones which may advantageously be reduced electrolytically to '(LB glycols in contact with the cathodes prepared in accordance with the present invention, the cells, conditions of electrolysisetc. are-disclosed inconsiderable'detail in our application Serial No. 449,718. filed July 5 4, 1942, now Patent 2,422,468, of which the present application is a continuation in part.

The foregoing description contains a limited number of embodiments but it will be understood that our invention is by no means limited to the specific details described since numerous variations are possible without departing from the scope of the subjoined claim.

What we claim and desire to secure by Letters Patent is:

A process for preparing cathodes suitable for the electrolytic reduction of organic compounds which comprises electrolytically depositing a thin film of lead weighing between about 0.02 and 0.5 gram per square dm. on copper from an aqueous electrolyte consisting essentially of 100 grams per liter of lead nitrate and from 1 to about 20 grams per liter of cupric nitrate at a current density of about one ampere per square dm.

THOMAS S. CHAMBERS. OBER C. SLOTTERBECK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Transactions of the Kansas Academy of Science, volume 45, pages 173 and 185 (1942).

Transactions of the Electrochemical Society,

20 volume 73, page 392 (1938). 

